Electroplating of aluminum and its alloys



United States Patent Office 3,284,323 ELECTROPLATLNG F ALUMINUM AND ITS ALLOYS Roberto Leloup, Bilbao, Spain, assignor to W. Canning & Company Limited, Birmingham, England No Drawing. Filed Aug. 30, 1962, Ser. No. 220,549 Claims priority, application Great Britain, Sept. 12, 1961, 32,724/61 14 Claims. (Cl. 204-38) This invention relates to the electroplating of aluminum and aluminum base alloys.

The immersion of aluminum or its alloys in aqueous alkaline solutions of sodium hydroxide containing sodium Zincate as a prior process to subsequent electroplating has been known previously and is termed a zincate dip. However, it has previously been found necessary in industrial practice to deposit copper or brass from an alkaline solution on to the immerision deposit of zinc formed from the zincate dip, before attempting to apply a nickel deposit from the standard acidic solutions normally employed for commercial nickel plating of steel, brass or copper articles. Otherwise only a poorly adherent nickel coat was obtained. The use of this additional copper or brass plating operation renders the process for nickel plating on to aluminum more complicated and costly than it otherwise would be.

The object of this invention is to eliminate the necessity of these copper or brass plating operations before the nickel plate is deposited, and to provide an alkaline solution containing an alkali metal zincate, e. g. sodium zincate, which solution gives an immersion deposit of zinc, on aluminum and its alloys, of a type which allows nickel to be plated directly on to the aluminum or its alloys from acidic nickel plating solutions as normally used commercially, while still giving nickel coatings which have excellent adherence.

The invention consists in an aqueous alkaline solution for use as a zincatc dip for aluminum, comprising a dissolved alkali metal zincate, a dissolved complex between nickel and cyanide ions, said nickel being present in an amount of at least 2 gms. per liter of solution, and a dissolved alkali metal hydroxide.

The complex Will not dissociate in the solution, so that therefore nickel-containing precipitates are not formed; however, it is instrumental in forming a coating, believed to be predominantly zinc, on suitably prepared aluminum, or aluminum alloy, surfaces when these are immersed in the solution according to the invention, and this coating, when e.g. nickel plated in commercial nickel plating solutions of either the dull or so-called semi-bright or bright types, provides a surface which allows a nickel coat to be deposited which has excellent adhesion to the metal underneath. No intermediate copper or brass plating is needed after this immersion treatment before the nickel plating, but only thorough rinsing, e.g. in water or a dip of very dilute sulfuric acid.

The nickel complex can either be formed in situ or prepared separately (by the interaction of a soluble nickel salt, e.g. nickel sulfate, nickel chloride or nickel nitrate and a complexing agent, e.g. potassium cyanide and/or sodium potassium tartrate) before addition to the alkaline solution of the alkali Zincate.

In one embodiment the invention consists in an aqueous solution containing:

G./l. Nickel sulfate hexahydrate 8 to 45 Zinc sulfate he-ptahydrate 20 to 70 Potassium cyanide to 30 Sodium hydroxide 80 to 180 In a preferred embodiment the solution contains:

3,284,323 Patented Nov. 8, 1966 G./l. Nickel sulfate (NiSO .6H O) 20 to 40 Zinc sulfate (ZnSO .7H O) 3 to 60 Potassium cyanide 7 to 13 Sodium hydroxide to Up to 45 g./l. of tartaric acid may be added to the above solution.

Alternatively, an amount of an alkali metal salt of tartaric acid (e.g. potassium hydrogen tartrate, sodium potassium tartrate (Rochelle salt) or disodium tartrate) supplying the same amount of tartrate ion as the above may be added.

When tartaric acid, or an acid tartrate, is added enough sodium hydroxide or other alkali hydroxide must be added to neutralize it. Other alkali metal cyanides than potassium cyanide may be used providing the'requisite quantity of cyanide ions are available for complex formation. Other alkali hydroxides can be used instead of sodium hydroxide in quantities to give the same resultant alkalinity.

Zinc oxide, zinc chloride and/or zinc nitrate can be used in place of zinc sulfate if the amount used is suflicient to supply the same quantity of zinc ions in soluble form expressed as metallic zinc.

Other chemicals which may be optionally added to the solution given above are water soluble cupric salts such as copper sulfate or nitrate, soluble ferric salts such as ferric nitrate or chloride and alkali metal nitrates.

These chemicals may be added individually or two or more used together provided the conditions given below are fulfilled. Cupric salts to provide up to 2 g./l. of cupric ions, or an equivalent amount of the copper-containing complex ions, in the solution may be added. They can be added either as a cupric salt solution in a small volume of water or, preferably, complexed by reaction with an alkali cyanide and/ or tartaric acid or an alkali tartrate before addition to the zincate solution. The ferric salts are used in proportions to give up to 1 g./ 1. of ferric ion, or an equivalent amount of iron-containing complex ions, in the zincate solution. Again the ferric salts may be added as a concentrated solution in water or else preferably complexed by reaction with tartaric acid or an alkali tartrate before addition to the zincate solution.

The alkali metal nitrates are added to give a concentration of nitrate ion of up to 2 g./l. per litre.

A suitable sequence of operations for treating a surface of aluminum or aluminum alloy prior to electrolytically depositing a metal thereon, comprises the steps of: alkaline cleaning of the said surface; acid etching of the said surface; immersion of the surface in the solution as described above in accordance with the invention; and rinsing of the surface with, e.g. water or dilute sulfuric acid.

It is greatly to be preferred and, for most practical purposes, essential to rinse the surface between the cleaning and etching and between the etching and immersion to prevent carrying over one solution to the next.

Preferably, the alkaline cleaning is effected by making the aluminum or its alloy cathodic in an aqueous solution containing trisodium phosphate and sodium hydroxide, e.g. a solution containing 18 oz./ gal. of sodium hydroxide and 4 OZ./ gal. of trisodium phosphate.

Preferably again, the acid etching is carried out in an aqueous solution of nitric acid, e.g. containing 50% In 31 modification, the surface is re-etched with acid after immersion in the solution according to the invention, and then re-immersed in the solution, being finally rinsed. Moreover, it may be of advantage to precede the process with a degreasing stage carried out in boiling trichlorethylene liquid and/or vapour.

Although this process is especially convenient for nickel I Alkali cleaning:

I Immersion in 15% by volume solution of plating, it may be used also when electro-plating with other metals, e.g. copper, zinc, brass, cadmium, silver, chromium, gold and tin.

The invention will be further described with reference to the following Examples, which are not to be construed as limiting the scope of the claims in any way.

Sodium hydroxide 115 Potassium hydrogen tartrate 40 was contacted with an aluminum surface which had been cleaned by making it a cathodic in a solution containing 18 02/ gal. sodium hydroxide and 4 oz./ gal. trisodium phosphate, followed by an acid etching in a 50% (v./v.) nitric acid solution.

Example 3.Examples 0] process sequences found satisfactory for aluminum and various aluminum base alloys N.B.: If the work is heavily soiled with grease and/or polishing composition, immersion of the aluminum or aluminum base alloy in boiling trichlorethylene liquid and/ or the vapour evolved from that liquid may be necessary to remove this gross contamination, prior to carrying out the sequences shown in the following Table.

The Roman numbers given are the order in which the operations were performed in the process sequence used, with the time for each operation in brackets. Intermediate rinsing of the aluminum or aluminum base alloy in cold running water is necessary between each of the operations listed in the above process sequences. Thorough rinsing eg in water or dilute sulfuric acid is also essential after the last operation noted above before immersion of the aluminum or aluminum base article having on its surface the novel zincate film into the nickel electroplating solution.

Aluminum Alloy Process Used Aluminum 99.99%) sfm 3.8. 1470 13.5. 1476 13.8. 1490 B.S. 1490 3.113 H.E.9. L.M.2. L.M.5.

Type of Zincate Solution used A C D E Immersion in 4 ozs./gall. sodium hydroxide at 160 F.

I see)..-

Cathodic treatment in solution contain- 18 ozs./gall. sodium hydroxide and 4 ozsJgall. trisodium orthophosphate at ambient temperature.

I (2 min.) I (1 min.)

Cathodic treatment in solution containing 3 ozs./gall. sodium hydroxide and 1 oz./ gall. trisodium orthophosphate at ambient temperature.

Acid etch:

Immersion in by volume solution 11 (1 min.) of nitric acid at ambient temperature.

II (30 see.

III (2 min)- II (2 min.). V (10 see)...

II (1% min.) II (1 min.) IV (30 Seen)" concentrated sulfuric acid at 150 F.

Zincate dip:

Immersion 1n zinc-ate solution contam- III (1 min.)

II (1 min).

[IV (30 sec.)

III (30 scc.) [VI (10 sec.)

Rinse:

Immersion in 0.5% by volume solution of concentrated sulfuric acid at ambient temperature.

A deposit formed on the surface of the aluminum which when plated in a conventional nickel-plating bath gave a good nickel coat with excellent adhesion to the underlying aluminum.

Example 2 The following solutions A F would be used in the method of Example 1:

Solution in ./l. Chemical g A B C D E F Nickel sulfate (NiSO .6H2O) 25 30 3O 30 30 30 Zin sulfate (ZnSO .7HzO) 40 4o 40 4s 4s 40 Sodium hydroxide 103 115 106 110 110 116 Potass um cyanide 30 10 11 11 11 11 Potassium hydrogen tartrate (K110 11406)..- 4O Sodium potassium tartrate (KNaC H O 45 40 40 40 Copper sulfate (CuSO4.5HzO) 5 5 Ferric chloride (F6Cl3.6H3O) 2 2 Sodium nitrate 1 1 Having thus described my invention what I claim as new and desire to secure by Letters Patent is:

1. An aqueous alkaline solution for use as a Zincate dip for aluminum, comprising a dissolved alkali metal zincate, a dissolved complex between nickel and cyanide ions, said nickel being present in an amount of at least 2 gms. per liter of solution, and a dissolved alkali metal hydroxide. I

2. An aqueous alkaline solution as claimed in claim 1, in which there is present from about 7 gms. to about 14 gms. of nickel per liter of solution.

3. An aqueous alkaline solution for use as a zincate dip for aluminum, comprising a dissolved alkali metal zincate, a dissolved complex between nickel and cyanide ions, said nickel being present in an amount of at least 2 gms. per liter of solution, a dissolved alkali metal hydroxide, and up to 45 gms. of tartaric acid per liter of the solution.

4. An aqueous alkaline solution for use as a zincate dip for aluminum, comprising a dissolved alkali metal zincate, a dissolved complex between nickel and cyanide ions, said nickel being present in an amount of at least 2 gms. per liter of solution, a dissolved alkali metal hydroxide and up to that amount per liter of the solution of an alkali metal tartrate which supplies the same concentration of tartrate ions as does 45 gms. per liter of tartaric acid.

5. An aqueous alkaline solution as claimed in claim 4, in which the said alkali metal tartrate is selected from the group consisting of potassium hydrogen tartrate, sodium potassium tartrate, and disodium tartrate.

6. An aqueous alkaline solution containing:

Gms./liter Nickel sulfate hexahydrate 8 to 45 Zinc sulfate heptahydrate to 70 Potassium cyanide 5 to Sodium hydroxide 80 to 180 7. An aqueous alkaline solution as claimed in claim 6 modified in that an alkali metal cyanide, other than potassium cyanide, is used in an amount that makes available in the said solution the same concentration of cyanide ion as does 5 to 30 gms./ liter of potassium cyanide.

8. An aqueous alkaline solution for use as a zincate clip for aluminum comprising a dissolved alkali metal zincate, a dissolved complex between nickel and cyanide ions, said nickel being present in an amount of at least 2 gms. per liter of the solution, a dissolved alkali metal hydroxide and a member of the group consisting of water-soluble cupric salts, water-soluble ferric salt-cupric salt complexes, and alkali metal nitrates.

9. An aqueous alkaline solution as claimed in claim 8 which contains a member of the group consisting of cupric sulfate, cupric nitrate, ferric nitrate, and ferric chloride.

10. An aqueous alkaline solution for use as a Zincate dip for aluminum comprising a dissolved alkali metal zincate, a dissolved complex between nickel and cyanide ions, said nickel being present in an amount of at least 2 gms. per liter of the solution, a dissolved alkali metal hydroxide and up to 2 grns. per liter of cupric ions.

11. An aqueous alkaline solution for use as a zincate dip for aluminum, comprising a dissolved alkali metal Zincate, a dissolved complex between nickel and cyanide ions, said nickel being present in an amount of at least 2 grns. per liter of the solution, a dissolved alkali metal hydroxide and up to 1 gm. per liter of ferric ions.

12. An aqueous alkaline solution for use as a zincate dip for aluminum comprising a dissolved alkali metal zincate, a dissolved complex between nickel and cyanide being present in an amount of at least 2 gms. per liter of the solution, a dissolved alkali metal hydroxide and up to 2 gms. per liter of nitrate ions.

13. In a process of nickel plating aluminum or aluminum-based alloys wherein prior to the electro-deposition of nickel the surface to be plated is immersed in an aqueous bath containing a dissolved alkali metal Zincate and a dissolved alkali metal hydroxide, the improvement which comprises adding a dissolved nickel cyanide complex to the aqueous bath prior to the immersion of the surface to be plated.

14. In a process of nickel plating aluminum or aluminum-based alloys wherein prior to the electro-deposition of nickel the surface to be plated is immersed in an aqueous bath containing a dissolved alkali metal zincate and a dissolved alkali metal hydroxide, the improvement which comprises adding a dissolved nickel salt and a source of cyanide ion successively to the aqueous bath to form a dissolved nickel cyanide complex in the aqueous bath prior to the immersion of the surface to be plated.

(1961-1962), Chemical Rubber Publishing Co., pages 448449.

Wernick, Finishing of Aluminum (1956), Robert Drapen, Limited, pages 422-444.

JOHN H. MACK, Primary Examiner. MURRAY TILLMAN, Examiner.

L. G. WISE, W. VAN SISE, Assistant Examiners. 

1. AN AQUEOUS ALKALINE SOLUTION FOR USE AS A ZINCATE DIP FOR ALUMINUM, COMPRISING A DISSOLVED ALKALI METAL ZINCATE, A DISSOLVED COMPLEX BETWEEN NICKEL AND CYANIDE IONS, SAID NICKEL BEING PRESENT IN AN AMOUNT OF AT LEAST 2 GMS. PER LITER OF SOLUTION, AND A DISSOLVED ALKALI METAL HYDROXIDE.
 14. IN A PROCESS OF NICKEL PLATING ALUMINUM OR ALUMINUM-BASED ALLOYS WHEREIN PRIOR TO THE ELECTRO-DEPOSITION OF NICKEL THE SURFACE TO BE PLATED IS IMMERSED IN AN AQUEOUS BATH CONTAINING A DISSOLVED ALKALI METAL ZINCATE AND A DISSOLVED ALKALI METAL HYDROXIDE, THE IMPROVEMENT WHICH COMPRISES ADDING A DISSOLVED NICKEL SALT AND A SOURCE OF CYANIDE ION SUCCESSIVELY TO THE AQUEOUS BATH TO FORM A DISSOLVED NICKEL CYANIDE COMPLEX IN THE AQUEOUS BATH PRIOR TO THE IMMERSION OF THE SURFACE TO BE PLATED. 